Method for controlling the viscosity of surfactant solutions

ABSTRACT

A method of recovering oil from a subterranean formation using an aqueous surfactant solution with a predetermined and controlled viscosity. The ability of the surfactant composition to impart viscosity to the aqueous solution is dependent upon the weight ratio of hydrocarbon oil in the composition to the surfactants in the composition. This oil/surfactant weight ratio must be maintained within the range of 0.05 to 0.40 to obtain a surfactant composition having the ability to impart a desired viscosity to its aqueous solution. The ratio can be maintained within this range and varied in value to change the viscosityimparting properties of the surfactant composition by adjusting the deoiling process during the manufacture of the surfactant composition or by adding hydrocarbon oil to the surfactant composition.

FIPWHZ UIlllBU DlalBS l'alelll l i Denekas 1 Aug. 21, 1973 [75]lnventor: Milton 0. Denekas, Houston, Tex. [73] Assignee: EssoProduction Research Company [22] Filed: Nov. 22, 1971 [211 Appl. No.:201,089

[52] U.S. Cl 166/252, 166/275, 166/273 REACTOR NEUTRALIZER PrimaryExaminerStephen J. Novosad Attorney-James A. Reilly et a1.

[ 5 7] ABSTRACT A method of recovering oil from a subterranean formationusing an aqueous surfactant solution with a predetermined and controlledviscosity. The ability of the surfactant composition to impart viscosityto the aqueous solution is dependent upon the weight ratio ofhydrocarbon oil in the composition to the surfactants in thecomposition. This oil/surfactant weight ratio must be maintained withinthe range of 0.05 to 0.40 to ob tain a surfactant composition having theability to im part a desired viscosity to its aqueous solution. Theratio can be maintained within this range and varied in value to changethe viscosity-imparting properties of the surfactant composition byadjusting the deoiling process during the manufacture of the surfactantcomposition or by adding hydrocarbon oil to the surfactant composition.

6 Claims, 1 Drawing Figure 2O l-" L. DEOILER l8 EVAPORATOR PAIENTEDMJBZIms 3.753.465

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l? ALIZER DEOILER I I4 REACTOR NEUTR MILTON O. DENEKAS INVENTOR.

ATTORNEY METHOD FOR CONTROLLING THE VISCOSITY OF SURFACTANT SOLUTIONSBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a process forrecovering oil from a subterranean formation.The invention also relates to a method for controlling the viscosity ofan aqueous solution used to recover oil from a subterranean formation.This invention further relates to a method of manufacturing surfactantshaving predetermined viscosity-imparting properties.

2. Description of the Prior Art The petroleum industry has recognizedfor many years that only a portion of the original oil in place in anoil reservoir can be produced by natural mechanisms. It is also wellknown that conventional methods of supplementing natural recovery arerelatively inefficient. Typically, a reservoir may retain, half itsoriginal oil even after the application of Errrently available methodsof secondary recovery. Accordingly, there is a continuing need forimproved recovery methods which will substantially increase the ultimateyield of petroleum from subterranean reservoirs.

Waterflooding is by far the most economical and wi ely practiced ofsecondary recovery methods. In such a process, water is introducedthrough injection wells to drive oil through the formation to offsetproducing wells. Much of the current work in secondary recoverytechnology has been directed toward improving the efficiency ofwaterflooding processes.

Surface active agents or surfactants are one class of materials whichhave been proposed for improving the efficiency of waterfloodingprocesses. Much of the oil that is retained in the reservoir after atypical water flood is in the form of discontinuous globules or discretedroplets which are trapped within the pore spaces of the reservoir. Ithas been suggested that, because the normal interfacial tension betweenthe reservoir oil and water is so high, these discrete droplets areunable to deform sufficiently to pass through narrow constrictions inthe pore channels. When surface active agents are added to the floodingwater, they lower the interfacial tension between the water and thereservoir oil and permit the oil droplets to deform and flow with theflood water. It is generally conceded that the interfacial tensionbetween the flood water and the reservoir oil must be reduced to lessthan 0.1 dyne/cm for effective recovery.

While conventional waterflooding and surfactant waterflooding may beeffective in obtaining additional oil from subterranean oil reservoirs,these processes have a number of shortcomings which reduce their abilityto recover oil. Foremost among these shortcomings is the tendency of theinjected fluid to finger through the reservoir and to bypass substantialportions of oil. In other words, as the injected fluid travels throughthe reservoir between injection wells and production wells, it contactsless than the total volume of the reservoir within the injectionwell-production well pattern. The fraction of the volume of thereservoir that is swept by injected fluid is termed the "sweepefficiency and is expressed as a percentage of the total reservoirvolume in the pattern. The sweep efficiency of a typical conventionalwaterflood or surfactant waterflood may typically be less than 75percent when the flooding operation reaches its economic limit. Thus,one quarter or more of the reservoir may not have been contacted by theinjected fluid at the end of the operation. The low sweep efficiency ofthese operations is usually explained by the fact that the injectedfluid has the ability to move through the reservoir at a much fasterrate than the oil which it is displacing. The fingering and bypassingtendencies of the injected fluid are due in part to its relatively lowviscosity.

The sweep efficiency of a flooding operation is dependent in part on themobility ratio of the flooding system. The mobility ratio is amethematical expression which relates fluid and formation rockproperties and which expresses the relative mobilities of the oil and ofthe driving water in a flooding operation. When the mobility ratioequation is applied to a typical waterflooding operation it is expressedas:

where M mobility of the oil in the reservoir in question;

M mobility of the driving water in the reservoir in question;

u,,, viscosity of the driving water;

t, viscosity of the oil;

K, relative permeability of the reservoir to the oil in the presence ofresidual water;

K relative permeability of the reservoir to water in the presence ofresidual oil;

This equation is perhaps best explained by stating that when themobility ratio of oil to water is equal to one the oil and the waterwill move through the reservoir with equal ease. When the mobility ratiois less than one, there will be a tendency for the water to bypass theoil and finger to the producing well. Naturally, when the mobility ratiois low the sweep efficiency will also be low.

The mobility ratio is related to the viscosities of the flooding fluidand the reservoir oil. The viscosities of reservoir crude oils can varyconsiderably. Some crudes might have viscosities as low as one or twocentipoises and others range up to a thousand centipoises or evengreater. However, the vast majority of reservoir crude oils which arecapable of being recovered by conventional or surfactant waterfloodinghave viscosities in the range of two to ten centipoises at reservoirtemperature and pressure. It should be readily apparent from themobility ratio equation that, if a surfactant waterflood with aviscosity of approximately one centipoise is used to displace oil havinga viscosity of five centipoises, there will be a tendency for the floodwater to finger through the reservoir oil. This, of course, will resultin a relatively poor sweep efficiency and a considerable portion of thereservoir oil may never be contacted by the surfactant flood water. Ithas, in fact, generally been noted that surfactant waterfloodingperforms less satisfactorily with viscous crude oils than withrelatively nonviscous oil.

A number of procedures have been suggested to date for improvingconventional and surfactant waterflooding to reduce the degree offingering and bypassing and to increase the sweep efficiency. Onesuggestion has been to increase the viscosity of the flood water byincorporating watersoluble, viscosity-imparting agents in the water.Materials that have been suggested for this purpose include a widevariety of gums, sugars, polymers and certain sulfonated hydrocarbons.While these materials are effective to an extent in increasing the viscosity of flood water they are also characterized by seriousdisadvantages. For example, some of the materials have a tendency toplug formations; some are relatively unstable; some have relativelylittle thickening effect; and none of the materials has the ability tolower the interfacial tension between the oil and water to desiredlevels. Additionally, many of these materials are quite expensive andtheir use is not feasible from the standpoint of economics.

A recent U.S. Patent Application Viscous Surfactant Waterflooding;Hopkins, Lederman, and Murray; (Ser. No. 148,127 filed May 28, 1971', acontinuation of Ser. No. 845,126, filed July 23, 1969; now abandoned)describes a new surfactant composition which has the dual ability ofincreasing the viscosity of the flood water and of radically loweringthe interfacial tension between the reservoir oil and flood water. Thesedual purpose surfactant compositions are effective oil recovery agents.They are inexpensive, they lower the interfacial tension to desirablelevels and they substantially increase the viscosity of the flood water.However, there is a need for means of controlling the viscosity ofsolutions of these surfactants while retaining a low interfacialtension. The viscosity of such solutions may be higher or lower than thevalue desired for particular application.

Generally speaking, an increase in the viscosity of the surfactant floodwater will improve oil recovery. However, an increase to a particulardesired level can be vitally important to the efficiency of therecovery. In other words, an increase in the viscosity of the floodwater may be beneficial, but unless this increase is brought within thedesired limits the recovery process cannot be conducted under optimumconditions. Where the increased viscosity is still too low there will bea tendency for that fluid to finger through the oil and to inefficientlysweep the reservoir. Where the increased viscosity is too high,excessive energy will be used in displacing the surfactant solutionthrough the reservoir. Moreover, where the viscosity of the surfactantsolution is higher than that of the fluid which displaces it, e.g.,flood water, there will be a tendency for viscous fingering to occur atthe trailing edge of the surfactant solution bank. Control of theviscosity level is therefore of paramount importance to an efficientdisplacement of reservoir oil.

It has previously been suggested that the viscosity of aqueous solutionsof these dual purpose surfactants can be controlled by varying the ratioof water-inso1uble/- pentane-insoluble sulfonates in the surfactantmixture. It has been found that when the weight ratio of thesesulfonates lies within the range of 0.01 to 0.30 the resultant aqueoussurfactant solution will have desirable viscosities and improved oilrecovery properties. While variation of the sulfonate ratio is aneffective method of controlling the viscosity of the surfactantsolution, the need exists for other methods of varying and controllingthe viscosity of aqueous solutions of these dual purpose surfactants.

SUMMARY OF THE INVENTION This invention relates to a method ofrecovering oil from a subterranean oil-bearing formation. A surfactantcomposition is prepared by sulfonating a hydrocarbon oil, preferably ina thin-film reactor. Deoiling of the reaction product is controlled tomaintain the weight ratio of oil to surfactant within the range of 0.05

to 0.40. Alternatively, additional quantities of the hydrocarbon oilfeedstock or an oil having a similar composition may be added to thereaction product.

The objects of this invention will be apparent from the followingdrawing and discussion of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a schematic flow diagramof a process for manufacturing the surfactants of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention has generalapplicability to adjusting and controlling the viscosity of aqueoussolutions of anionic surfactants. However, the results of the practiceof this invention are most pronounced when used with the dual purposesulfonated surfactants previously discussed. Therefore, this inventionwill be described with primary reference to this preferred class ofsurfactant composition.

It has been found that the viscosity-imparting properties of thedual-purpose surfactant compositions are related, at least in part, tothe relative amounts of certain components in the surfactant product.These viscositycontrolling components are the oil and the surfactants,both water-insoluble and pentane-insoluble sulfonates. These componentswill be defined and discussed in greater detail hereinafter. It hasfurther been found that the relative amounts of these constituents canbe varied, and thus the viscosity-imparting properies of the surfaceactive agents can be controlled by regulating the operating conditionsduring the manufacture of the product.

Prior to discussing the method of determining the constituents of thesurfactant product, the effect of the oil/surfactant weight ratio on theviscosity of an aqueous solution of the product and the method ofcontrolling the value of this ratio within desirable limits, it may behelpful to review the general method for producing these dual purposesurfactants as disclosed in U.S. Patent No. 148,127. The method ofmanufacturing these dual purpose surfactants is set forth in detail inthat patent application and that disclosure is incorporated by referenceherein. However, for background and convenience, a general descriptionof the method of producing such surfactants will be given.

These dual purpose surfactants are prepared by sulfonating certainhydrocarbon feedstocks under specified sulfonating conditions. Thepreferred source of feedstock hydrocarbons is a 700-1 F boiling rangefraction of a petroleum crude. Such fractions are known generally in theart of petroleum refining as lube oil distillates" and may be identifiedby the ASTM Standard Method of Distillation, D 1160-61.

As set forth in Patent Application Ser. No. 148,127, the feedstocks canbe obtained from a variety of sources. The source may be a virgindistillate, an unrefined petroleum crude, or a distillate which has beensubjected to further refining steps such as hydrofining, dewaxing, orsolvent extraction. All of these feedstocks are satisfactory forproducing the dual purpose surfactant so long as they contain at leastten percent by volume of the sulfonatable constituents present in a700-1100F boiling range fraction of a petroleum crude.

The FIGURE will further illustrate the process of manufacture of thesedual purpose surfactants. Turning to this drawing, a suitable petroleumfeedstock in line and a sulfonating agent, such as gaseous S0 and air,in line 1 1 are introduced into the sulfonation reactor 12 where aportion of the petroleum feed is converted into sulfonic acids. Wastegases containing primarily sulfur dioxide, sulfur trioxide, oxygen andnitrogen are discharged by way of vent line 13. The temperature of thereactor effluent may vary broadly from 150 to 375F. This reactortemperature will be governed primarily by the viscosity of thefeedstock, the reactor design and the desired flow rate of feedstockthrough the reactor. Generally, reaction temperatures of from about 250to 325F are preferred.

The gaseous sulfur trioxide is introduced into the hydrocarbon feedstockin an inert diluent gas, such as air or nitrogen. Sulfor trioxide willpreferably be from about 5 to 8 percent by volume of the total gaseousfeed volume. This percentage is not critical, however, and may vary fromas low as 0.5 percent up to approximately 25 percent by volume. Thediluent gas should be dried before the sulfur trioxide is introduced toprevent reaction between sulfur trioxide and water vapor.

The treat ratio for the process expresses the relationship between thequantity of sulfur trioxide and the quantity of hydrocarbon feedintroduced into the reactor and is expressed as the number of pounds ofsulfur trioxide per 100 pounds of hydrocarbon feed. This treat ratio mayvary between 5 and 30 pounds of S0 per 100 pounds of hydrocarbonfeedstock but preferably is about 20 pounds per hundred.

The average period of time that the sulfonatable material remains in thereactor is termed the residence time". Short residence times arepreferable; approximately 1 second to 5 seconds generally producedsuperior products. However, the residence time depends primarily on theconfiguration of the reactor and may vary broadly. In certain instances,the residence time may be several minutes without adverse results.

The hydrocarbon feed stream and the gaseous S0 may be preheated beforeintroduction into the reactor. These increased temperatures will speedthe reaction and lower the viscosity of the hydrocarbon feedstock. Thepreferred feedstocks for producing these dual purpose surfactants arenormally maintained at a tempera ture of from 150 to 210F.

Thin-film reactors are the preferred type of sulfonation reactor toproduce these dual purpose surfactants. Such reactors include thefalling film reactors such as shown in British Pat. No. 1,111,208 andGerman Pat. No. 1,195,299. Wiped-film reactors are another type of thisgeneral class and are typified by the reactor disclosed in U. S. Pat.No. 3,427,342. These thin-film reactors including the falling film andwiped-film types have the common feature of means for producing a verythin film of hydrocarbon liquid within the reaction zone. Gaseous S0,,in the diluent gas passes across this film and induces turbulence in thefilm to promote rapid reaction and rapid transfer of heat from theinterior of the reactor to its exterior.

After the desired degree of sulfonation the reactor effluent iswithdrawn by means of line-14 and comprises a mixture of sulfonic acids,unreacted hydrocarbon feedstocks, and minor amounts of unreacted gaseousS0 The mixture is then introduced into neutralizer 15 where it isneutralized by an aqueous solution of basefrom line 16. Neutralizationof these sulfonic acids is a conventional technique and many bases bothorganic and inorganic may be employed. The preferred bases forneutralization are ammonia and sodium hydroxide. Generally, enough baseis added to bring the pH to about 9-1 1 and neutralization temperaturesnormally range from 60210F.

The neutralized product is withdrawn by means of line 17 and introducedinto the deoiling chamber 18. In the preferred method of this invention,the operating conditions in the deoiling chamber 18 are controlled toremove a desired amount of unreacted oil from the neutralized productand to leave a desired quantity of unreacted oil in the neutralized,deoiled sulfonation product.

The removal of unreacted oil from surfactants produced from petroleumfeed streams is conventional and a number of deoiling methods may besuitably employed in the practice of this invention. However, a singlesolvent process using isopropyl alcohol is preferred. isopropyl alcoholis introduced through line 19 to the deoiling chamber 18. The isopropylalcohol will mix with the neutralized product and create two phases. Theupper phase will primarily consist of unreacted oil with minor amountsof isopropyl alcohol, sulfonated surfactants and water. The lower phasewill contain sulfonated hydrocarbons, most of the added isopropylalcohol, inorganic salts, minor quantities of unreacted oil, and waterresulting from the aqueous base solution used in the neutralizationstep. The total volume of alcohol-water should be approximately equal toor slightly greater than the volume of the other constituents of themixture. 1f insufficient water is present from the neutralization togive this equal volume additional water may be introduced with thealcohol through line 19.

The quantity of alcohol employed in the deoiling procedure can be usedto control the amount of unreacted oil remaining with the alcohol-waterphase. When more unreacted oil is desired in the sulfonation product,the proportion of isopropyl alcohol should be increased. When less oilis desired, the quantity of alcohol should be decreased. The quantity ofalcohol to be employed in a specific instance will vary with a number offactors including the total quantity of unreacted oil present in theneutralized product, the temperature of the product stream in thedeoiling chamber, and the desired quantity of unreacted oil in theneutralized deoiled product. With these and similar operating conditionsin mind, one of ordinary skill in the art can easily determine theproper quantity of alcohol to be used in a specific application.

After phase separation has occurred in the deoiling chamber, theunreacted oil will be withdrawn through line 20. The second liquid phasecontaining alcoholwater, sulfonated hydrocarbons, inorganic salts, andthe desired quantities of unreacted oil will be withdrawn through line21 and discharged into the evaporation stage 22. In the evaporationstage, water and alcohol are removed through lines 23, and the dried,neutralized crude product is discharged through line 24.

The constituents of this dual purpose surfactant product can besegregated and identified using standard solvent extraction andqualitative analysis techniques. For convenience and clarity, theseconstituents will be defined herein in terms of this standard solventextraction process. Any remaining water and alcohol is first removedfrom the product by drying. The dried product is then placed in asolution of 85 volume percent isopropyl alcohol and 15 volume percentwater. A portion of the sample will be soluble in the solution and aportion will be insoluble. The insoluble portion is filtered from thesolution and set aside for further analysis. The solution containing thealcohol-water soluble fraction is then dried to remove the alcohol andwater. The dry residue is then placed in pentane. A portion of thisresidue is insoluble in pentane and is separated from the solution byconvenient means such as filtration. Qualitative analysis reveals thatthis pentaneinsoluble constituent is essentially a water-solublehydrocarbon sulfonate. The pentane-soluble fraction is a hydrocarbon oilhaving a mass spectrum similar to that of the original feedstock. Thisfraction appears to be oil which did not react in the sulfonation step.

The fraction of the original sample which is not soluble in theisopropyl alcohol-water solution is then placed in water. A portion issoluble and qualitative analysis reveals that this fraction isessentially inorganic salt, such as Na SO which was produced by thereaction of excess S and the base during the neutralization step. Theconstituent which is insoluble in water is essentially a hydrocarbonsulfonate having a relatively high equivalent weight. It should be notedthat the water-insoluble sulfonate can be solubilized in water in thepresence of the water-soluble, pentane-insoluble sulfonate. Also theunreacted oil which would otherwise be insoluble in water is soluble inthe presence of the sulfonated hydro-carbons.

An analysis of a typical dual purpose surfactant in the form of thedried neutralized product is:

TABLE I Dried Product Composition Constituent Weight PercentPentane-insoluble sulfonate 68.4 Oil l4.l Inorganic Salt (sodiumsulfate) 11.0 Water-insoluble sulfonate 6.5

Total: l00.0

As was previously noted, it has been found that the viscosity-impartingproperties of these surfactant compositions are related in part to therelative quantities of oil and surfactant present in the surfactantproduct composition. Moreover, it has been found that these quantitiesmust be kept within limits to produce a satisfactory product for oilrecovery. For convenience, the relative quantities of oil and surfactantwill be referred to herein as the weight ratio between the oil in thecomposition to the surface active agents in the composition, i.e., thepentane-insoluble and water-insoluble sulfonates. It has been found thatoil/surfactant ratio should be no lower than 0.05 and should be nohigher than 0.40 by weight. As will be shown hereinafter theviscosity-imparting properties of the composition are unsatisfactorilylow unless the oil/surfactant ratio is kept within the stated limits.

To illustrate the relationship between the oil concentration of thesurfactant composition and its viscosityimparting properties, thesurfactant having the composition illustrated in Table I was treated toremove the inorganic salt and the unreacted oil. The inorganic salt wasremoved using conventional desalting methods, and the oil was thenremoved from the sample using a dialysis extraction technique. Dialysisis a microfiltration method based on the differing diffusionalcharacteristics of substances across a semi-permeable membrane. In theseparation of the oil from the surface active agents of the surfactantcomposition, a Soxhlet extractor was employed. This conventionalextraction apparatus has a separation chamber for holding separationsolvents such as pentane, a siphon discharge leading from the separationchamber into a heating flask, a condenser-refluxer for liquefying thevapors which are distilled within the heating flask, and a samplecontainer disposed within the separation chamber. The sample containeror Soxhlet thimble is an elongated cup of a porous and permeablepaper-like material for holding the surfactant sample. The surfactantsample is placed in a sealed latex membrane which is disposed within theSoxhlet thimble. The thimble itself extends above the liquid level ofthe solvent and the sample within the latex membrane is below the liquidlevel line.

In operation, the condensed pentane within the separation chamberpermeates the thimble and latex membrane and permits diffusion of theoil from the sample and into the pentane. When the pentane level reachesthe siphon height, it automatically discharges into the heating flaskwhere it is distilled and introduced into the condenser-refluxer. Thedistilled pentane liquefies within the condenser and is reintroducedinto the separation chamber. The condensed pentane and oil pass freelythrough the latex membrane whereas the other components of the sulfonateproduct do not. Since the oil has a much higher boiling point than thepentane, it concentrates within the heating flask. This analyticaltechnique is an effective separation method; qualitative analysis of theseparated oil shows that it is essentially free of sulfonatedhydrocarbons.

These dialyzed and desalted surfactant samples were then used toillustrate the effect of oil concentration on the viscosity-impartingproperties of the surfactant. An undialyzed whole surfactant sample wasadded at a concentration of 2 weight percent to water containing 3percent by weight sodium carbonate. The viscosity of this surfactantsolution was then measured in a Wells- Brookfield Micro Viscometer MOdelLVT at a shear rate of 1 l5 reciprocal seconds and a temperature ofapproximately 25C. The viscosity of this whole sample was 10.6centipoise. The surfactant was then desalted and dialyzed in a mannerpreviously described to remove the salt and the unreacted hydrocarbonfeedstock from the sample.

A number of dialyzed surfactant solutions were prepared in a similarmanner and various concentrations of oil from the original feedstockwere added to these solutions. The viscosities of each of thesesolutions were then measured in the Wells-Brookfield Micro Viscometer.The results of these tests are shown in Table ll.

TABLE I] Apparent Viscosity Measured at Reciprocal Seconds SurfactantSample Dialyzed Sample Dialyzed Sample 5 wt% Oil Dialyzed Sample 10 Wt%Oil Dialyzed Sample 20 wt% Oil 1 Dialyzed Sample 40 wt% Oil It should benoted at this point that the viscosities tabulated in Table II areapparent viscosities. These solutions have non-Newtonian fluid behavior.They are pseudo-plastic since their apparent viscosity will decreasewith an increase in shear rate, and they are thixotropic since whensheared at a given rate their apparent viscosities will decrease withtime.

It is readily apparent from the results tabulated in Table II that theviscosity of an aqueous solution of these dual purpose surfactants canbe readily controlled by controlling the quantity of oil contained inthe surfactant solution. When the oil concentration is less than percentby weight of the dialyzed sample, the viscosity of the sample is lessthan 3 centipoise. When the oil concentration is increased to percent,the viscosity of the solution quadruples.

The effect of the oil in increasing the viscosity of the solution isparticularly surprising in view of the very minor quantities employed.In the maximum quantity stated the oil is added at a concentration of 40percent of the dialyzed sample which is added to the aqueous solution ata concentration of 2 percent. In other words, the oil is only 0.8 weightpercent of the resultant aqueous solution. Moreover, the maximumviscosity is noted where the oil concentration is only 0.4 percent ofthe aqueous solution.

While the foregoing investigations were conducted by adding quantitiesof the original feedstock to the dialyzed sample, it should be notedthat comparable results have been obtained by adding extracted,unreacted oil to its dialyzed sample. These tests showed correspondingresults. When the oil concentration was less than 5 percent by weight ofthe dialyzed sample there was essentially no increase in viscosity. Theviscosity of the aqueous solution rose to a peak with increasing oilconcentrations and then decreased as further quantities of oil wereadded.

While it is preferred to control the quantity of oil in the surfactantcomposition by controlling the operating conditions in the deoilingchamber, it is contemplated that other procedural steps can be employedto vary the oil concentration. For example, the dried neutralized crudeproduct can be produced and then analyzed to determine the quantity ofoil in the product. If the oil quantity is too low to producesatisfactory viscosities, additional feedstocks can be used to raise theoil concentration to a desired level. If the oil concentration is toohigh, additional extraction procedures can be employed to reduce the oilconcentration to within desired limits.

in many instances, it will be preferred to maintain the oil/surfactantratio at a level such that aqueous surfactant solution will have amobility ratio of approximately one with the reservoir oil. To achievethis viscosity, it may be necessary in some cases to add a specificviscosity increasing agent such as a heteropolysaccharide to thesurfactant solution. This invention may be useful even in such aninstance since less of the specific viscosity increasing agent will beneeded to achieve the desired viscosity level. This reduction in thequantity of specific viscosity increasing agent will, of course, improvethe economics of the oil recovery method.

ln other instances, it may be desired to employ a surfactant solutionwhich has a mobility ratio of less than one with respect to thereservoir oil. Although the sweep efficiency of the surfactant solutionwould be reduced in such an instance, this lower mobility ratio may bemore economical than adding an additional viscosity increasing agent.Again, the benefits of this invention can be realized in such aninstance. Although the mobility ratio of the surfactant solution withrespect to oil may not be equal to approximately one, the practice ofthis invention will result in a more viscuous surfactant solution. Eventhough the sweep efficiency may not be at an optimum, it will beimproved by the practice of this invention. In certain instances soundreservoir engineering may dictate that the mobility ratio of thesurfactant solution with respect to oil should be less than one. Thiscondition could arise where it is desired to reduce fingering bydisplacing flood water into the surfactant solution. The practice ofthis invention will permit the operator to achieve this desiredviscosity level.

A typical operation in which this invention might be carried out isillustrated by the following example:

A petroleum reservoir is water flooded in the conventional manner to aresidual oil saturation of about 30 percent of the reservoir porevolume. Using standard reservoir techniques, it is determined that asolution containing approximately 2 weight percent surfactant and havinga viscosity of approximately 11.5 centipoises at reciprocal secondscould satisfactorily displace oil from this particular reservoir. Asurfactant solution is prepared by reacting a petroleum feedstockboiling within the range of 700-1 l0OF in a wiped-film reactor. Theconditions within the deoiling chamber of the sulfonation process areadjusted to produce a dried neutralized crude product having anoil/surfactant ratio of approximately 0.20. The product is subsequentlyconcentrated and shipped to the field location. The surfactant mixtureis added at a concentration of 2 percent by weight based on thepentane-insoluble sulfonate concentration to watercontaining 3 percentby weight sodium carbonate. The viscosity of the resultant mixture isapproximately 11.5 centipoise at l 15 reciprocal seconds. The volume ofthe solution injected into the reservoir is approximately 30 percent ofthe reservoir pore volume within the area to be swept by the solution.The injected surfactant solution is followed by a 30 percent pore volumebank of flood water containing 0.04 weight percent of aheteropolysaccharide viscosity-increasing agent having a viscosity ofapproximately 3 centipoises at a shear rate of 230 reciprocal seconds.The surfactant in thickened water banks is then displaced toward aproducing well by injection of oilfield brine. Displaced reservoir oilis recovered from the producing well.

The principle of the invention and the best mode in which it iscontemplated to apply that principle have been described. It is to beunderstood that the foregoing is illustrative only and that other meansand techniques can be employed without departing from the true scope ofthe invention defined in the following claims.

What is claimed is:

1. A method for recovering oil from a subterranean oil bearing formationby injecting an aqueous solution of hydrocarbon surfactant into saidformation which comprises determining a viscosity level for the aqueoussurfactant solution to effectively displace the oil, injecting into theformation an aqueous solution of hydrocarbon surfactant having anoil/surfactant weight ratio which is within the range of 0.05 and 0.40and at a ratio within the range which is sufficient to impart thedetermined viscosity level to the surfactant solution, displacing thesurfactant solution through the formation, and recovering oil from theformation.

I; l v v 4. A method as defined by claim 1 further comprising addingviscosity increasing agent to the surfactant solution to furtherincrease the viscosity of the solution.

5. A method as defined by claim 1 wherein the surfactant solutionexhibits non-Newtonian fluid behavior.

6. A method as defined by claim 5 wherein the non- Newtonian fluidbehavior is pseudo-plastic and thixotropic.

1? I i i

2. A method as defined in claim 1 wherein the concentration of saidsurfactant in said solution is approximately two percent by weight.
 3. Amethod as defined by claim 1 in which said surfactant is prepared bycontacting a hydrocarbon feedstock containing at least ten percent byweight of the sulfonatable constituents present in a 700*-1100*F boilingrange fraction of a petroleum crude with gaseous SO3 in a thin-filmreactor and said oil is an unreacted portion of said hydrocarbonfeedstock.
 4. A method as defined by claim 1 further comprising addingviscosity increasing agent to the surfactant solution to furtherincrease the viscosity of the solution.
 5. A method as defined by claim1 wherein the surfactant solution exhibits non-Newtonian fluid behavior.6. A method as defined by claim 5 wherein the non-Newtonian fluidbehavior is pseudo-plastic and thixotropic.